In the general form of a turning machine, a workpiece is turned by a spindle having an axis of rotation. A machining tool, such as a cutting tool, is mounted in a tool holder that is movable through a range of positions adjacent the axis of rotation. The workpiece can be shaped into the desired form by selectively moving the tool holder with respect to the axis of rotation to change or alter the point of engagement of the tool with the workpiece. In modern turning machines the motion of the tool holder is controlled by a numerical control unit that contains a stored program with instructions for the movement of the tool holder.
Increased productivity can be realized by equipping the turning machine with more than one spindle to perform more than one simultaneous turning operation. In this form of turning machine, a plurality set of tool holders, equal in number or sets to the number of spindles, are mounted on a common slide that moves each tool holder through a range of positions with respect to the axis of rotation of its respective spindle. The machining operation may then be accomplished by moving the entire slide through a sequence of operations under the control of a stored program in a numerical control unit.
A problem that is invariably associated with automatically controlled turning machines is the need for tool offset. More specifically, the position of a tool will frequently need to be recalibrated or offset to accommodate certain operating parameters affecting proper tool position. Some of these operating parameters are, for example, tool wear; tool displacements due to thermal and structural deflections of the turning machine support structure; tool presetting errors; tool insert size errors; and programming and/or processing errors in the control unit. When the influence of one or more of these parameters causes the tool position to be outside the tolerance limits, then tool offset must be made to bring the tool back into proper positional relationship with respect to its associated spindle.
The problem of tool offset or recalibration becomes even greater when multi-spindle turning machines are used. More specifically, each of the plurality of tools on the multi-spindle turning machine may require independent offset or recalibration due to such factors as uneven tool wear or errors in tool size or presetting. In this instance, each tool holder mounted on the common slide must be movable with respect to every other tool holder on the slide. In the specific example of a turning machine having first and second spindles, a first tool holder is fixedly mounted on the slide, and a second tool holder is movably mounted on the slide with respect to the fixed position of the first tool holder.
The problem becomes increasingly complex where machining operations are done in two dimensions with respect to the axis of rotation of the spindle. The two-dimensional or biaxial turning machine requires a first, compound slide to move in a first direction with respect to the spindle axes of rotation, and a second, cross slide, mounted in sliding relation to the compound slide, to move in a second direction transverse to the direction of motion of the compound slide. First and second auxiliary slides are mounted in sliding relation on the cross slide and are movable parallel to the compound slide and cross slide, respectively. Each of the auxiliary slides carries a tool holder or turret with a plurality of tool holders.
The patent to Dormehl et al, U.S. Pat. No. 3,754,178 proposes a control system for tool offset in a multi-spindle, single axis turning machine. However, its disclosure is concerned only with a single axis of a turning machine, and does not address the further problems which may require bidirectional, independent tool offset in a biaxial turning machine. Moreover, the Dormehl et al control system requires rather elaborate signal processing circuitry to accomplish independent tool offset. Specifically, it requires two signal comparisons, a signal summing and a signal inversion to effect an independent offset of one tool holder with respect to the other.
It is an objective of the present invention to provide a multi-spindle, biaxial turning machine that includes a control system for providing independent tool offset in either of two dimensions. Moreover, another objective of the invention is to provide a relatively simplified machine and control system design that does not require elaborate signal processing circuitry.